Nuclear reactors sometimes include pressure vessels that are partially or wholly filled with water. One or more probe-type heaters are adapted to be inserted into the vessel and selectively operated to raise the temperature of the water therein. However, the temperature of the vessel water may vary widely. For example, if cold water is admitted to fill the vessel, the temperature of the vessel water may quickly drop. Indeed, the temperature of such vessel water may vary from about 38.degree. C. (100.degree. F.) to about 343.degree. C. (650 .degree. F.). At the same time, the pressure within the vessel may vary from about 0 bar (0 psig) to about 174 bar (2200 psig).
Often, the pressure vessel has a hemi-spherical head at its lower end. A plurality of depending tubular sleeves are welded to the vessel wall and surround holes provided therethrough. These sleeves provide a means or mechanism by which the probe-type heaters may be inserted into the vessel. There may be on the order of 40-100 of these heater sleeves on the vessel.
It is also desirable to know the temperature gradient of the vessel water. To this end, it is known to replace one of the heaters with a heater facsimile temperature detector ("HFTD"). These devices are so-named because the detector is also a probe-type device that outwardly appears to have substantially the same shape as the insertable probe-type heater element. Since they simulate the outward appearance of the heater elements, they are denominated "heater facsimiles". However, internally they have a plurality of axially-spaced temperature detectors, such as thermocouples, operatively arranged to indicate temperature of the vessel water at various points along the length of the probe.
If the dynamic response of a prior art HFTD could be increased, the result would be greater accuracy and precision in its measurements. It is important that the operator know the operational condition of the vessel at all times. Hence, it would be generally desirable to provide an improved HFTD that affords the capability of enhanced dynamic response, particularly to rapid and abrupt changes in the temperature of the vessel water.